In a gas fired water heater, the burner is mounted beneath the lower head of the water storage tank and the waste gases of combustion are discharged through a flue that extends centrally through the tank. The hot waste gases passing upwardly through the flue increase the rate of heat transfer to the water contained within the tank.
To prevent corrosion of the steel tank, the inner surfaces of the tank, including the cylindrical shell, the upper and lower heads and the flue, are normally glass coated. It has also been recognized in the prior art that other types of corrosion resistant materials such as zinc, aluminum and magnesium can be applied to the inner surfaces of the tank. Such coating procedures are for example described in U.S. letters Pat. Nos. 2,490,978 and 2,566,138. Methods of application have included hot dipping, spraying and painting. In addition, the prior art describes tanks which have been provided with non-corrosive liners such as aluminum as described in U.S. letters Pat. No. 2,993,617.
Further, a sacrificial anode is normally suspended in the upper head of the heater and projects downwardly into the tank. The anode is composed of a metal anodic to steel, such as magnesium, aluminum or zinc, and acts to cathodically protect any areas of the steel tank which may be exposed through defects in the glass coating or at the tank fittings.
The most conventional practice currently in fabricating a water heater is to initially weld the upper head to the upper end of the shell, with the inner surfaces of the preassembled shell and upper head then being coated with glass. As a separate operation, the flue is welded within an opening in the lower head and the outer surface of the flue as well as the upper surface of the lower head are similarly coated with glass.
Prior to firing the glass at an elevated temperature, the glass coating is brushed away from the inner surface of the annular flange bordering the opening in the upper head, as well as from the upper end of the flue so that when the flue is subsequently assembled within the opening in the upper head, a weld can be made directly from the steel upper head to the steel flue. Similarly, prior to firing, the glass coating along the lower edge of the shell and along the peripheral edge of the lower head is brushed away so that a weld can be made directly between the shell and the lower head. However, with such conventional practice, there is an area of exposed steel at the joint between the flue and the upper head as well as at the joint between the lower head and the shell.
Further, the upper heads of water heaters typically have a downwardly depending peripheral flange which is press fitted for assembly purposes within the upper end of the shell. In effecting such assembly, the upper head is forced through a sizing ring and disposed within the upper end of the shell so that the press fit is provided therebetween. Typically, the free end of the upper head flange flares inwardly slightly so that a crevice is created between the outer surface of the upper head flange at its lower extremity and the side wall of the shell. Such crevices can be on the order of 0.015 inches or 15 mils at their opening. In the conventional practice of applying a glass coating to this area, it has been found that glass does not effectively flow around and cover sharp corners and does not tend to effectively fill or bridge and seal the aforesaid crevice. In addition, since glass is relatively inelastic, it tends to crack more easily than metallic coatings for example. Thus, in the conventional water heater using glass as an interior coating, or metallic coatings generally which do not effectively seal the aforesaid assembly welds, a substantial portion of the cathodic efficiency of the anode or anodic surface is expended in protecting the exposed steel at these joints. Not only is there an inefficient use of the anode when the assembly welds are not effectively sealed, there also is the drawback that no effective corrosion barrier has been provided and that no hydraulic seal has been provided with respect to the weld areas.
U.S. Pat. No. 4,113,600 discloses a method of minimizing the exposed steel at the joint between the flue and the flange on the upper head. In accordance with the aforementioned patent, an anodic metal is sprayed in the form of a small strip onto the exposed steel at the upper end of the flue which overlaps the glass coating on the flue. On assembly, the anodic metal coating is press fitted into engagement with the upper head to seal off exposed steel at the joint between the flue and the upper head. However, this arrangement has the drawback that manufacturing assembly tolerances on the order of plus or minus one-half inch can exist when assembling the subassembled flue/lower head with the subassembled shell/upper head whereby the vertical positioning of the press fitted metal coating in U.S. Pat. No. 4,113,600 may not be effective. Further, in this arrangement, with the remainder of the flue being glass coated, no substantial cathodic protection is provided by the flue.
Due to the conventional central disposition of a flue, prior art anodes have been necessarily mounted in an off-center position which reduce their cathodic effectiveness in certain areas of the tank. Put another way, the off-center location of a prior art anode results in it not being equidistant from the entire inner surface of the cylindrical shell whereby varying degress of cathodic protection are provided to the various tank areas.
Another problem arises from the presence of exposed steel in the area of mounting a prior art anode in the upper head. The conventional manner of mounting an anode is to weld a spud to the outer surface of the upper head bordering an opening therein. The upper end of the anode is then threaded within the spud and extends downwardly into the tank. However, the edge of the upper head bordering the opening, through which the anode extends, often is inadequately coated with glass, and a portion of the spud, which is normally formed of steel, is exposed through the opening. Thus the upper end of the anode can be prematurely consumed in protecting the edge of the upper head bordering the opening as well as in protecting the exposed spud. This results in a "necking down" condition of the upper end of the anode adjacent the upper head. Extreme "necking down" can result in the exposure of the steel core wire of the anode and the exposure of the steel wire will further decrease the effectiveness of the anode in protecting exposed steel surfaces of the tank.
Moreover, in the conventional cathodically protected water heater, the core wire is exposed at the lower end of the anode and the exposed end of the steel core wire will result in increased consumption of the lower end of the anode, causing the lower end of the anode to assume a bullet-like shape.